would prefer something free, don't like stuff that munches a huge chunk of cpu.
imo the best filter that there is in the world of software is the opposite of both these things. Antares Filter VST is

. If you can get it for Mac in an, erm, "try before you buy" manner then I recommend it.
Also, OhmForce do a nice little freebie harmonic filter called Frohmage, available on their website and can produce a variety of nice warm tones.
My sort of stock, workhorse filter is the low/high pass pair on the MS20fx plugin in the Korg Legacy pack. The filters on that thing are enough to give a man a serious hard-on.